As a light source that displays a projected image on a screen such as a data projector or a rear surface projection type television image receiver, an extra-high pressure mercury lamp has been hitherto used, however, in recent years, laser is proposed from the viewpoint of a mono-chromaticity and the life of the light source.
Further, in order to supplement a red color light that is insufficient in the ultra-high pressure mercury lamp, a light source using together the ultra-high pressure mercury lamp and a red color laser is also proposed.
However, when the laser is used as the light source, speckle noise in particles due to the coherent property of a laser beam occurs in the projected image so that the quality of the projected image cannot be avoided from being deteriorated.
JP-A-2000-206449 (patent document 1) and JP-A-2006-047421 (patent document 2) propose a projection type display device that reduces the speckle noise.
FIG. 17 is a block diagram of an image display device disclosed in the patent document 1. Lights outputted from a semiconductor laser 81 are changed to parallel lights by a collimator lens 82, and then, pass through a transparent optical element 83. The transparent optical element 83 is made of optical glass having a refractive index of n and formed in the shape of a stairs of N steps with an depth of Δt.
The lights passing through the transparent optical element 83 pass through a light converging lens 85 via a lens array 84 composed of N elements respectively corresponding to the steps of the transparent optical element 83. The lights passing through the light converging lens 85 are spatially modulated by a spatial modulator 86 and projected on a screen 88 through a projection lens system 87.
Since the lights passing through parts of the transparent optical element 83 respectively include optical path differences mutually integer times as large as Δt, the lights reaching the screen form a collection of the lights whose phases are spatially shifted and a coherence property is deteriorated. Thus, the occurrence of the speckle noise is suppressed.
FIG. 18 is a perspective view of a spatial polarization control element disclosed in the patent document 2 that is formed in a vertical stripe form in which for instance, phase difference plate areas 91 provided with phase difference plates as ½ wavelength plates and transmitting member areas 92 provided with transmitting members are alternately arranged.
Since the speckle noise of the light passing through the phase difference plate areas 91 is not correlated with the speckle noise of the light passing through the transmitting member areas 92, the speckle noise formed on a display screen on which the light passing through the phase difference plate areas 91 is superimposed on the light passing through the transmitting member areas 92 is reduced.
However, in the device disclosed in the patent document 1, when the step Δt is increased in order to provide a sufficient optical path difference, a problem arises that the device itself is enlarged.
Further, in the device disclosed in the patent document 2, since a stray light is generated in the boundary between the phase difference plate area and the transmitting member area, when the areas are finely divided to reduce the speckle noise, a problem arises that the stray lights are increased to deteriorate a contrast of a display image.